PROJECT SUMMARY/ABSTRACT This application proposes ancillary studies to the NIDDK-Biliary Atresia Research Consortium (BARC). Biliary atresia, the most common cause of neonatal cholestasis, results from a fibro-inflammatory obstruction of extrahepatic bile ducts. Despite nearly uniform progression to end-stage cirrhosis, the variable rate of progression and response to treatment suggest the existence of unrecognized phenotypes that are based on the biology of disease. In view of the multifactorial pathogenesis of disease, we previously combined patient- based functional genomics and mechanistic studies in experimental biliary atresia and discovered a central role for proinflammatory immunity in disease pathogenesis. In preliminary studies for this application, we built a biliary atresia platform containing clinical, laboratory, histological, and genome-wide expression data for patients enrolled in a BARC prospective observational multi-center study and a corticosteroid trial. Initial analysis of this platform identified novel molecular subtypes that are biologically linked to histological features and relevant to the clinical outcome. Here, we propose a logical continuation of these studies with the overall aim of defining the biological basis for disease phenotypes and clinical outcomes of children with biliary atresia. To this end, we will pursue three aims: 1) To discover novel molecular subtypes of biliary atresia and define key regulatory networks, 2) To determine transcriptional predictors of favorable response to anti-inflammatory treatment, and 3) To define the cellular mechanisms regulated by inflammatory genes in biliary atresia. To achieve these aims, we will expand the biliary atresia platform with clinical/laboratory and gene expression data for patients enrolled in the BARC prospective observational study and the corticosteroid trial. We will then analyze the platform to validate the novel inflammatory and fibrosing subtypes and demonstrate their relevance to clinical course or progression of disease. To obtain insight into molecular networks regulating individual subtypes, we will apply functional and comparative genomics and use cell- and organ-based experimental systems to investigate how specific networks regulate pathogenesis of disease. Using the inflammatory signature, we will determine whether it is associated with an increased response rate to corticosteroid treatment. Finally, we will use complementary in vitro systems to explore cellular mechanisms of epithelial injury using mononuclear cells from patients with both subtypes. Collectively, the proposed experiments will create unparalleled opportunities to re-define the clinical spectrum of disease and to individualize treatments according to the patients' biological makeup.